In the basic vapor-compression refrigeration cycle, the temperature of a working fluid, or refrigerant, is reduced below the environmental temperature by an expansion process. Energy is then transferred as heat from the space to be cooled to the working fluid in an evaporator. To reuse the refrigerant, it is repressurized to raise its temperature above the environmental temperature. Heat is then transferred from the refrigerant to the environment in a condenser. The cycle then repeats.
Chloroflourocarbons (CFCs) are often used as the working fluid in vapor-compression refrigeration systems. However, in recent years, CFC pollution and particularly the destructive effect of CFCs on the earth's ozone layer have become of increasing concern. A major source of CFC pollution is the automobile air conditioning system, which typically uses the CFC freon as the working fluid. The CFCs are prone to leak out of the air conditioner both during the useful life of the air conditioner and when the vehicle is discarded. The state of Vermont and cities in California have even banned the use of automobile air conditioners which use CFCs as the working fluids.
Refrigerant systems have been devised in which the refrigerant is water and which use a steam ejector rather than a compressor in the cooling cycle. Railroad passenger cars were cooled with a steam ejector system during the era of steam locomotion. This system was practical at the time, since there was a ready source of high pressure steam and the size of the system was not critical. The steam ejector system was replaced by other types of air conditioning systems when diesel locomotives replaced steam locomotives.
In the 1970s, a solar powered refrigeration system was proposed and analyzed, although never constructed. In this system, solar energy was used to boil a working fluid. The output from the boiler was a high velocity, subsonic (Mach number less than 0.2) vapor which entered an ejector system. Water was found to be the working fluid that gave the highest coefficient of performance.
Water has several disadvantages as a working fluid, however, particularly for use in automobiles. Water is liable to freeze in winter in temperate climates, causing damage to the system. Also, water systems require a low operating pressure in the evaporator and must be fairly large, so that they are difficult to fit into small vehicles.
Ejector refrigeration systems powered by automobile engine waste heat have been used. However, these prior art systems use a CFC refrigerant and do not account for the problems in the transient response characteristics of the ejector system due to fluctuations in the waste heat output of the engine, such as during engine start-up.